Lighting Design Glossary

Inverse Square Law

Also sometimes referred to as square law.

(Term of photometry/radiometry)

The measured luminous flux density (or radiant flux density)
from a point light source decreases along any line from the source.
It falls in inverse proportion to the square of the relative distance.
An illuminance measurement 2 meters from the light source will be 1/4 of
the measurement 1 meter from the source.

The law holds because the energy from the source radiates in all directions, in
effect spreading itself over the surface of a sphere.
At a greater distance,
the same amount of energy is spread over a larger surface, and thus any
one area of the surface will receive less. The increase in surface area is
proportional to the square of the radius.

This model of propagation is based on a point source, radiating equally in all
directions.
Real light sources rarely behave exactly like point sources in this
respect, especially for very short distances relative to the size of the source.
With larger distances, however, the inverse square law gives a sufficiently
accurate approximation for most practical purposes, at least within the bounds
of a specified solid angle.